Description of the Hubbard U terms used in the density functional theory (DFT) calculations contained in the Materials Project (MP) database.
It is well-known that first principles calculations within the local density approximation (LDA) or generalized gradient approximation (GGA) lead to considerable error in calculated redox reaction energies of many transition metal compounds. This error arises from the self-interaction error in LDA and GGA, which is not canceled out in redox reactions where an electron is transferred between significantly different environments, such as between a metal and a transition metal or between a transition metal and oxygen or fluorine. Extensive discussion of this issue can be found in the following works. [1-4]
In the Materials Project, we have calibrated
values for many transition metals of interest using the approach outlined in Wang et al.'s work . At the present moment,
values have only been calibrated for transition metal oxide systems.
values were calibrated for the following elements:
. The choice of systems to which we apply
was largely determined by our experience and by systematic benchmarking. It is very likely that we will expand calibration of
values to more chemical systems in the future.
Note that for fluorides, the
value gets set to the one calibrated from the oxide system, although in principle our architecture allows different
values to be set for oxides and fluorides respectively.
values were obtained by fitting to experimental binary formation enthalpies as described in Wang et al.'s work. This method is simple yet accurately reproduces phase stabilities. A least squares method of obtaining the correct
value was used, as follows:
- 1.For each non-overlapping formation energy reaction considered, we find the region where the formation energy error passes zero. For thesystem, this includes the following:
- 2.For each formation energy region identified, we fit the linear equationto the finalrange. In the case of, we will have two sets of.
- 3.We find the U value that minimizes the sum of square Error / Redox.
- 4.In the case of, we get avalue of 3.25.
The full list of U values used is described in the table below. For oxides and fluorides containing any of the elements, only GGA+U calculations are performed.
The U values are calibrated for phase stability analyses, and should be used with care if applied to obtain other properties such as band structures. Also, the U values depend on the pseudopotential used. Further, typically, U values should be site specific, however in our approach, U values were applied to all sites with an element listed above, and only to the d-orbitals. A discussion of the pseudopotentials used in the Materials Project can be found here.
- 1.Shyue Ping Ong
: F. Zhou, M. Cococcioni, C. A. Marianetti, D. Morgan and G. Ceder. First-principles prediction of redox potentials in transition-metal compounds with LDA+U. Physical Review B, 2004, 70, 235121. doi:10.1103/PhysRevB.70.235121
: M. Cococcioni, S. de Gironcoli, Linear response approach to the calculation of the effective interaction parameters in the LDA+U method. Physical Review B, 2005, 71, 035105. doi:10.1103/PhysRevB.71.035105
: A. Jain, G. Hautier, S. P. Ong, C. Moore, C. Fischer, K. A. Persson, & G. Ceder. Formation enthalpies by mixing GGA and GGA + U calculations. Physical Review B, 2011, 84(4), 045115. doi:10.1103/PhysRevB.84.045115
: M. Wang, A. Navrotsky Enthalpy of formation of LiNiO2, LiCoO2 and their solid solution, LiNi1-xCoxO2, Solid State Ionics, vol. 166, no. 1-2, pp. 167-173, Jan. 2004.